Polymer composite using shear thickening fluid

ABSTRACT

Disclosed are a shear thickening fluid (STF) and a polymer composite comprising a core filled with a shear thickening fluid containing silica particles dispersed in polyethylene glycol; an inner capsule layer formed of an emulsifier surrounding the shear thickening fluid; and an outer capsule layer formed of a thermosetting resin surrounding the inner capsule layer.

TECHNICAL FIELD

The present invention relates to a polymer complex comprising a shearthickening fluid.

BACKGROUND ART

Suspension or liquid dispersion systems in which fine particles aredispersed in a continuous-phase solvent are divided into the followingtwo categories according to shear rate characteristics: Newtonian fluidsthat show a constant viscosity regardless of a change in the shear rate;and non-Newtonian fluids whose viscosity changes when the shear ratechanges.

A shear thickening fluid (STF), a kind of non-Newtonian fluid, is afluid such as a colloidal suspension which contains solid particlesdispersed in a liquid dispersion medium and which reversibly changesfrom a liquid state to a solid state due to its rheological propertieswhen the shear stress or shear rate thereof increases to rapidlyincrease the viscosity.

The shear thickening fluid is usually in a liquid state, and changes toa solid state when a sudden shock is externally applied thereto. Due tothis property, studies focused on impregnating the shear thickeningfluid into fibers to provide bullet-resistant or stab-resistantmaterials are currently being actively conducted.

The shear thickening fluid is generally prepared as a sol-typedispersion or suspension by mixing nano-sized silica particles as solidparticles with the polar solvent polyethylene glycol as a dispersionmedium. In order to bullet-resistant or stab-resistant performance usingthe shear thickening fluid, it is required to increase the rate offilling of inorganic particles in the shear thickening fluid or the rateof impregnation of the shear thickening fluid in fibers. If the rate offilling of inorganic particles or the rate of impregnation of the shearthickening fluid is increased as described above, there will be aproblem in that the impregnated shear thickening fluid flows down toform an agglomerate to thereby greatly reduce rather than increasebullet-resistant performance.

In addition, because the shear thickening fluid is in a liquid state,the use thereof alone may be limited, and the shear thickening fluidwill be likely to flow down with the passage of time when it isimpregnated into fiber. In addition, when the shear thickening fluid ismixed with a polymer material to prepare a composite, the uniformity ofdistribution of the shear thickening fluid can decrease. For thesereasons, there is a need for studies to solidify the shear thickeningfluid so as to widen the range of application thereof and process theshear thickening fluid so as to be uniformly distributed throughout apolymer complex.

DISCLOSURE Technical Problem

Accordingly, the present invention has been made keeping in mind theabove-described problems occurring in the prior art, and an object ofthe present invention is to provide a capsulate a shear thickening fluidto prepare powder, thereby widening the range of application of theshear thickening fluid and, at the same time, increasing the uniformityof distribution of the shear thickening fluid in a polymer composite tothereby increase the shear thickening ability and elasticity of theshear thickening fluid.

Technical Solution

In order to accomplish the above object, the present invention providesa shear thickening fluid (STF) microcapsule comprising: a core filledwith a shear thickening fluid containing silica particles dispersed inpolyethylene glycol; an inner capsule layer formed of an emulsifiersurrounding the shear thickening fluid; and an outer capsule layerformed of a thermosetting resin surrounding the inner capsule layer.

The emulsifier forming the inner capsule layer of the shear thickeningfluid microcapsule may be styrene maleic anhydride.

The thermosetting resin forming the outer capsule layer of the shearthickening fluid microcapsule may be melamine resin, urea resin, phenolresin, a melamine-urea copolymer, a melamine-phenol copolymer, or epoxyresin. Preferably, it may be melamine resin.

In the present invention, the silica particles may be fumed silicaparticles. The fumed silica particles may have a bimodal particle sizedistribution in which the size of small-diameter particles is 50-100 nmand the size of large-diameter particles is 110-150 nm. The fumed silicaparticles may be contained in an amount of 5-30 wt % based on the totalweight of the shear thickening fluid.

In the present invention, the silica particles may be spherical silicaparticles, and the spherical silica particles may be contained in anamount of 20-80 wt % based on the total weight of the shear thickeningfluid.

The present invention also provides a method for preparing a shearthickening fluid (STF) microcapsule, comprising the steps of: mixingsilica particles and polyethylene glycol at a stirring speed of 120-5000rpm to prepare a shear thickening fluid (STF); emulsifying the shearthickening fluid by an emulsifier to form an inner capsule layer; addinga polycondensation polymer to the inner capsule layer, and curing theadded polycondensation polymer to form an outer capsule layer; andadding a cationic coagulant to the aqueous solution containing the outercapsule layer to solidify the solution, followed by filtration of asolid component.

In the method of the present invention, the emulsifier may be an aqueoussolution of styrene maleic anhydride.

The polycondensation polymer is an aqueous solution of melamineformaldehyde.

The step of emulsifying the shear thickening fluid to form the innercapsule layer may comprise performing stirring in an emulsificationvessel at a speed of 50,000 rpm or more to form emulsion particleshaving a particle size of 1-5 μm.

The present invention also provides a polymer composite containing theshear thickening fluid microcapsule. The polymer composite may beselected from among soft polyurethane foam (PUF), a silicon elastomer,and rubber.

Advantageous Effects

The shear thickening fluid microcapsule according to the presentinvention is prepared by capsulating a conventional liquid shearthickening fluid to prepare powder. Thus, according to the presentinvention, the range of application of the shear thickening fluid iswidened, and the shear thickening fluid is distributed uniformly in apolymer composite so that the shear thickening property and elasticitythereof will be improved.

DESCRIPTION OF DRAWINGS

FIG. 1 is an SEM photograph of shear thickening fluid microcapsulescontaining fumed silica particles, prepared in Example 1 of the presentinvention.

FIG. 2 is an SEM photograph of shear thickening fluid microcapsulescontaining spherical silica particles, prepared in Example 2 of thepresent invention.

MODE FOR INVENTION

To achieve the above object, the present inventors have conductedstudies to capsule a shear thickening fluid, thereby completing thepresent invention.

A shear thickening fluid (STF) microcapsule according to the presentinvention comprises: a core filled with a shear thickening fluidcontaining silica particles dispersed in polyethylene glycol; an innercapsule layer formed of an emulsifier surrounding the shear thickeningfluid; and an outer capsule layer formed of a thermosetting resinsurrounding the inner capsule layer.

In the present invention, the silica particles may be any known silicaparticles such as fumed silica particles or spherical silica particles,which are used in shear thickening fluids. Preferably, the silicaparticles are fumed silica particles. The fumed silica particles areformed by hydrolysis in flames at a temperature of 1,000° C. or higheras shown in the following reaction equation:

SiCl₄+2H₂+O₂→SiO₂+4HCl  Reaction Equation 1

Primary particles made in flames are connected to one another due tomutual collision to form secondary particles which formthree-dimensional aggregates (agglomerates). The primary particles ofthe fumed silica particles are very small in size, are amorphous, andhave a large surface area.

Such fumed inorganic particles have advantages over spherical silicaparticles or colloidal silica particles in that they have a very smallprimary particle size, are lightweight, have a large surface area, andimprove the lightweight properties of products. In addition, such fumedparticles have a very big advantage in terms of costs. On the contrary,unlike spherical silica particles in which the size of primary particlesis the same as the size of secondary particles, the fumed silicaparticles have a problem in that they are very difficult to disperseuniformly, due to their aggregation. However, in a previous study, thepresent inventors found that the fumed silica particles can beeffectively controlled to be dispersed and the shear thickening property(bullet-resistant property) thereof is improved, as disclosed in KoreanLaid-Open Patent Publication No. 10-2012-0122387.

The fumed silica particles may be distributed non-uniformly and may havea bimodal particle size distribution. This reason is believed to bebecause the fumed silica particles can be filled at an increased ratewhile being distributed non-uniformly in a dispersion medium and easilyform hydroclusters. This can maximize the frictional force in a threadcoming-out phenomenon that occurs upon the collision of a bullet,thereby improving the bullet-resistant property and preventing the fumedsilica particles from being separated from bullet-resistant materials.

In the shear thickening fluid comprising the silica particles having abimodal particle size distribution, the silica particles may have aparticle size distribution in which the size of small-diameter silicaparticles is 50-100 nm and the size of large-diameter silica particlesis 110-150 nm. Preferably the silica particles may preferably have aparticle size distribution in which the size of small-diameter silicaparticles is 60-80 nm and the size of large-diameter silica particles is110-120 nm. Herein, the small-diameter silica particles and thelarge-diameter silica particles are preferably contained in the shearthickening fluid at a weight ratio of about 6:4 to 9:1, more preferably7:3. If the silica particles having a particle size difference of about10-100 nm between the small-diameter silica particles and thelarge-diameter silica particles as described above are distributednon-uniformly in an organic solvent, preferably in the form of a bimodalparticle size distribution, the effect of the non-uniform distributionof the fumed silica particles can be maximized, thereby improving thebullet-resistant property at the same weight and improving thelightweight property.

The silica particles are preferably contained in an amount of 5-30 wt %,more preferably 10-20 wt %, based on the total weight of the shearthickening fluid.

As a dispersion medium for the silica particles, polyethylene glycolwhich is a polar organic dispersion medium is used. The silica particlesare very stably dispersed in the polar dispersion medium, because thesilica particles have a large amount of a hydroxyl group on the surface.

The shear thickening fluid of the present invention may be prepared bystirring each component using a mixing device or a homogenizer. Herein,the stirring speed is preferably about 120-3000 rpm for non-uniformparticle distribution. If the stirring speed is less than 120 rpm, thesolid particles will be difficult to mix with the dispersion medium, andthe stirring speed is more than 3000 rpm, it will be difficult toachieve a non-uniform silica particle distribution such as theabove-described bimodal particle size distribution.

The spherical silica particles have a high dispersion stability inpolyethylene glycol, and thus may be contained in an amount of about20-80 wt % based on the total weight of the shear thickening fluid.

The prepared shear thickening fluid is emulsified by an emulsifier toform the inner layer of the shear thickening fluid microcapsule.

The emulsifier that is used in the present invention is preferably anaqueous solution of styrene maleic anhydride, but is not limitedthereto.

Because styrene maleic anhydride is highly viscous, an aqueous solutioncontaining styrene maleic anhydride at a concentration of 3-8% may beused as the emulsifier.

After the emulsifier is added to the shear thickening fluid, the mixtureis introduced in an emulsification vessel and stirred at a speed of5,000-13,000 rpm to form emulsion particles having a particle size ofabout 1-5 μm.

After the inner capsule layer is formed of the emulsifier, an excessiveamount of melamine resin, urea resin, phenol resin, a melamine-ureacopolymer, a melamine-phenol copolymer or an epoxy resin precursor,preferably, a melamine resin precursor, is added to the emulsifiedsolution, and then allowed to react at a temperature of about 80-100° C.for about 8 hours so as to be cured to form an outer capsule layer. Acationic coagulant is added to the prepared microcapsule aqueoussolution to solidify the solution, after which the solidified materialis filtered, dried and then powdered, thereby obtaining microcapsulepowder containing the shear thickening fluid according to the presentinvention.

Hereinafter, the present invention will be described in detail withreference to preferred examples and comparative examples. The examplesof the present invention may, however, be embodied in different formsand should not be construed as limited to the examples set forth herein.Rather, these examples are provided so that this disclosure will bethorough and complete, and will fully convey the scope of the presentinvention to those skilled in the art.

Materials

-   -   Fumed silica particles: Aerosil 200 (A200, Evonik Degussa        Corporation) particles were used which are hydrophilic silica        particles having a specific surface area (BET) of 200±25 m²/g,        an average primary particle size of 12 nm and an equivalent        property of about 2.5 [SiOH] group/nm² or 0.84 mmol/g.    -   Dispersion medium: polyethylene glycol (molecular weight: 200;        Sigma-Aldrich Corporation) was used. It was dried under reduced        pressure at 50° C. for 8 hours to remove water.

Example 1

A. 108 g of fumed silica particles were added to 498 g of PEG (molecularweight: 200), and the mixture was stirred using a stirring motor(M8GA6M, Panasonic) and an anchor-type impeller at a speed of 150 rpmfor 6 hours, thereby preparing a silica particle dispersion. Theprepared silica particle dispersion was allowed to stand at roomtemperature for 24 hours to remove bubbles, thereby preparing a shearthickening fluid (STF).

B. 150 g of a 5% aqueous solution of styrene maleic anhydride as anemulsifier was added to the prepared shear thickening fluid, and themixture was introduced into an emulsification vessel and emulsified bystirring at 10,000 rpm for 10 minutes to form an inner capsule layer(emulsion particles: 2-3 μm).

C. An aqueous solution of melamine formaldehyde was added to theemulsified solution (emulsion particles: 2-3 μm) in an amountcorresponding to about 20 times the weight of the emulsifier, and thenallowed to react at 90° C. for 8 hours, thereby forming an outer capsulelayer.

D. A cationic coagulant was added to the resulting aqueous solution, andthe obtained solid component was filtered, dried, and then powdered,thereby preparing shear thickening microcapsules.

FIG. 1 is an SEM photograph of the prepared shear thickening fluidmicrocapsules containing fumed silica particles.

Example 2

Shear thickening fluid microcapsules were prepared in the same manner asdescribed in Example 1, except that 750 g of spherical silica particleswere added instead of the fumed silica particles.

FIG. 2 is an SEM photograph of the prepared shear thickening fluidmicrocapsules containing spherical silica particles.

1. A shear thickening fluid (STF) microcapsule comprising: a core filledwith a shear thickening fluid containing silica particles dispersed inpolyethylene glycol; an inner capsule layer formed of an emulsifiersurrounding the shear thickening fluid; and an outer capsule layerformed of a thermosetting resin surrounding the inner capsule layer. 2.The shear thickening fluid microcapsule of claim 1, wherein theemulsifier is styrene maleic anhydride.
 3. The shear thickening fluidmicrocapsule of claim 1, wherein the thermosetting resin is an aqueoussolution of melamine formaldehyde which is melamine resin.
 4. The shearthickening fluid microcapsule of claim 1, wherein the thermosettingresin is one selected from the group consisting of urea resin, phenolresin, a melamine-urea copolymer, a melamine-phenol copolymer, and epoxyresin.
 5. The shear thickening fluid microcapsule of claim 1, whereinthe silica particles are fumed silica particles.
 6. The shear thickeningfluid microcapsule of claim 5, wherein the fumed silica particles have abimodal particle size distribution.
 7. The shear thickening fluidmicrocapsule of claim 6, wherein the particle size of small-diametersilica particles in the fumed silica particles having the bimodalparticle size distribution is 50-100 nm, and the particle size oflarge-diameter silica particles in the fumed silica particles is 110-150nm.
 8. The shear thickening fluid microcapsule of claim 5, wherein thefumed silica particles are contained in an amount of 5-30 wt % based onthe total weight of the shear thickening fluid.
 9. The shear thickeningfluid microcapsule of claim 1, wherein the silica particles arespherical silica particles.
 10. The shear thickening fluid microcapsuleof claim 9, wherein the spherical silica particles are contained in anamount of 20-80 wt % based on the total weight of the shear thickeningfluid.
 11. A method for preparing a shear thickening fluid (STF)microcapsule, comprising the steps of: mixing silica particles andpolyethylene glycol at a stirring speed of 120-5000 rpm to prepare ashear thickening fluid (STF); emulsifying the shear thickening fluid byan emulsifier to form an inner capsule layer; adding a polycondensationpolymer to the inner capsule layer, and curing the addedpolycondensation polymer to form an outer capsule layer; and adding acationic coagulant to aqueous solution containing the outer capsulelayer to solidify the solution, followed by filtration of a solidcomponent.
 12. The method of claim 11, wherein the emulsifier is anaqueous solution of styrene maleic anhydride.
 13. The method of claim11, wherein the polycondensation polymer is an aqueous solution ofmelamine formaldehyde.
 14. The method of claim 11, wherein the step ofemulsifying the shear thickening fluid to form the inner capsule layercomprises performing stirring in an emulsification vessel at a speed of50,000 rpm or more to form emulsion particles having a particle size of1-5 μm.
 15. A polymer composite containing the shear thickening fluidmicrocapsule of claim
 1. 16. The polymer composite of claim 15, whereinthe polymer composite is soft polyurethane foam (PUF), a siliconelastomer, or rubber.